Copper foil-clad circuit board substrates are usually produced by lamination molding in which a copper foil is superimposed on an insulating support and they are molded together. As such copper foils, 105 .mu.m, 70 .mu.m, 35 .mu.m, 18 .mu.m and 12 .mu.m thick copper foils and the like are mass-produced by the electrolytic process, and those formed on a support such as an aluminum foil and having a thickness of 5 .mu.m, 9 .mu.m, etc. are also produced. Further, there are copper foils produced by the rolling process, but those having thicknesses of 35 .mu.m or less have not yet been commercially available, since the smaller the thickness, the higher the costs of such copper foils, because of its production method.
In the case where the copper foil used in lamination molding is too thin, extreme difficulties are encountered in superimposing the copper foil on an insulating support because such a copper foil is prone to become wrinkled, and also in that the wrinkles occur by the lamination molding after superimposing. The copper foil formed on a support such as an aluminum foil may eliminate this problem, but this copper foil has disadvantages that it costs too much and use of this copper foil necessitates the step of peeling the support such as an aluminum foil from the copper foil before a printed circuit is formed.
Therefore, there has actually been not easy to mass-produce a double-side copper-clad substrate having different thicknesses of two copper foils, such as a double-side copper-clad substrate having provided on one side thereof a thin copper foil prepared by the electrolytic process or a copper foil having a thickness of less than 20 .mu.m prepared by the rolling process, and having provided on the other side thereof a copper foil having a thickness of, for example, 20 .mu.m to 105 .mu.m.
On the other hand, there has been a known method in which a copper foil-clad circuit board substrate is subjected to preliminary etching with an etching liquid containing copper chloride, iron chloride, etc. in the processing of printed circuit board substrates, to remove a thin surface layer of the copper foil, and the resulting copper foil is used in the manufacture of printed circuit boards. However, in the case where the thickness of the surface layer of the copper foil removed in the preliminary etching exceeds a few microns, or where a copper-clad substrate having a size as large as 500.times.500 mm or larger is subjected to etching, the thickness variation of the resulting etched copper foil becomes too large, and hence this method has been regarded as disadvantageous in that it cannot produce thin copper foil-clad circuit board substrates which can be practically used.
For example, JP-A No. 62-200796 discloses a method in which a circuit board substrate overlaid with a copper foil having an average thickness of 18 .mu.m or more is subjected to mechanical abrasion, electrochemical polishing or chemical etching to reduce the thickness of the copper foil to 12 .mu.m or less, thereby to prepare an extremely thin copper foil-clad circuit board substrate. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) In Example 1 of the specification of this prior art reference, there is a description to the effect that a copper foil-clad circuit board substrate having a size of 340.times.340 mm and an average copper foil thickness of 18 .mu.m was subjected to etching with an etching liquid comprising cupric chloride and hydrochloric acid to etch the whole surface of the copper foil, thereby obtaining a thin copper foil-clad circuit board substrate having an average copper foil thickness of 10 .mu.m.
However, this method has the following disadvantages The present inventors reproduced this method, as. will be described in Comparative Example 1 described hereinafter, by the use of a conventional spray-type etching process, and found that at the time when only 5 copper-clad laminates had been subjected to etching to remove the surface layers of the copper foils by 6 .mu.m on an average, the thickness variation on the basis of an average thickness was .+-.3.5 .mu.m and that on the basis of a desired thickness was .+-.4.8 .mu.m, these variations being almost equal to the thickness removed by the etching. This tendency became strong with the increase in the thickness removed by etching or in the number of products produced. Hence, the above method cannot produce extremely thin copper foil-clad circuit board substrates which have a thickness variation sufficiently low for commercialization. Further, other methods disclosed in this prior art reference, including mechanical abrasion and electropolishing, gave similar results, showing difficult commercialization of this process. Moreover, in the case where the whole surface of a copper foil was etched with a usual etching liquid to prepare a thin copper-clad circuit board substrate, as described in Example 1 of this prior art reference, the resulting surface of the remaining copper foil was highly active and, hence, corroded in a short time in the atmosphere, although there is no problem if the substrate obtained is immediately used in the manufacture of circuits therefrom.
Furthermore, it has been extremely difficult to uniformly etch the surfaces of the copper foils of single-side copper-clad laminates having a thickness of 0.3 mm or less, because such laminates suffer from deformation due to curling etc.